Field of the Invention
The invention relates to a memory cell having at least one MOS transistor. The transistor contains a source, a first gate electrode, a second gate electrode, a drain and a channel. The first gate electrode is insulated and can contain an electric charge and a control voltage can be applied to the second gate electrode. The source, the drain and the channel are formed by differently doped regions of a semiconductor substrate and at least one dielectric layer which forms a gate dielectric is situated between the semiconductor substrate and the gate electrodes.
The invention furthermore relates to a method for fabricating such a memory cell.
Such a memory cell is described in U.S. Patent No. 5,242,848. In this case, the first gate electrode extends in a planar manner on a dielectric layer and has a tip. The second gate electrode has a plurality of regions, a lower region being disposed on the same dielectric layer as the first gate electrode and an upper region of the second gate electrode is disposed above the first gate electrode in regions. This configuration produces locally a particularly large electric field gradient on the surface of the first gate electrode. A tip effect promotes Fowler-Nordheim tunneling. Fowler-Nordheim tunneling involves charge transport through an insulator. The charge transport through the insulator is generally dependent to a great extent on the applied electric field. In the case of Fowler-Nordheim tunneling, the electric current density j has the particular dependence j=C.sub.1.times..epsilon..sup.2 exp (-.epsilon..sub.0 /.epsilon.)where .epsilon. is the electric field strength and C.sub.1 and .epsilon..sub.0 are constants dependent on a effective mass of the charge carriers and a height of a barrier layer. By virtue of the high electric field density, the memory cell of the generic type can be electrically erased in a particularly simple manner.
It has been shown, however, that, for feature sizes of 0.25 .mu.m or less, the memory cell does not have the necessary reliability for memory cells.